1. Field of the Invention
The present invention relates to the field of audio electronics, and in particular to cables for the transmission of line-level analog audio signals.
2. Description of the Related Art
High quality high-fidelity components used for music reproduction such as preamplifiers, amplifiers, digital-to-analog converters and tuners often employ analog signaling to convey the music signal from one component to the next.
Signal degradation can occur in interconnects that convey signals between these components due to the interaction of the signal conductors with the driving and receiving components. A number of measurable interconnect parameters can contribute to this degradation including: inductance, capacitance, dielectric absorption, dielectric loss, bandwidth and susceptibility. It is desirable to optimize these parameters in order to minimize degradation of the signal as it is conveyed from one component to the next.
An optimum interconnect should achieve low capacitance and inductance, uniform low-loss dielectric (preferably air), some form of shielding or common-mode noise rejection, have the ability to be terminated with standard RCA plugs or XLR connectors and have mechanical flexibility.
Line-level single-ended signaling generally involves a component containing a driver with an impedance in the range of 7-300 ohms which drives a signal over a cable or interconnect which terminates at a receiver in a second component with an impedance in the range of 10-75K ohms. Due to the small output signal voltages and high input impedances, the resulting currents are very small. Because of the high impedance of the receiver and the relatively high output impedance of the driver, capacitance and inductance in the interconnect can limit the bandwidth of the transmission system. It is therefore an objective to achieve the lowest possible inductance and capacitance in a single-ended interconnect. To accomplish this, tradeoffs must be made between: 1) spacing the conductors close together as in a twisted-pair to achieve low inductance, and 2) separating the conductors with air spacers to achieve low capacitance. As the inductance is reduced, the capacitance increases. Likewise, when the capacitance is reduced the inductance generally increases. There are also practical physical limits to spacing the conductors in order that the cable remain flexible and convenient to use in a system. An optimum balance of capacitance and inductance combined with uniform low-loss dielectric is required to achieve a theoretically ideal interconnect.
Balanced line-level interconnects also benefit from low capacitance and inductance although the drivers used for balanced signaling are generally lower impedance, typically in the range of 7-50 ohms, and the load is standardized with a termination of 600-1K ohms, which makes the interconnect less sensitive to these parameters.
Non-uniform or lossy dielectrics surrounding the conductors of the interconnect can cause phase shifts and noise in the signal that vary with frequency. These phenomena are generally manifested as what has been described as "veils" over the reproduced music material as reported from empirical study. These phase shifts and self-generated noise can also cause a defocusing of the image in the reproduced music program. This phenomena has been attributed to several mechanisms including conductor stranding effects, "skin effects" and imperfect dielectrics which are lossy and exhibit dielectric absorption. Dielectric absorption and dielectric losses can affect the signal by not allowing the interconnect to completely charge and discharge with transient musical passages.
In order to minimize the inductance, it would seem to be advantageous to utilize large gauge conductors in interconnects, however there is a practical upper limit because of "skin effects". Skin effects are present due to the wide band of frequencies that are present in most high-fidelity music material. The currents associated with the low frequencies tend to travel deeper within the cross-section of the conductor than the high frequencies which tend to travel more on the outer surface skin of the conductor. This is known as skin-effect. Skin effect is a function of the conductor material and geometry. Skin-effect can cause the music signal to become attenuated at some frequencies and distorted or smeared due to changing phase as the frequency changes. To minimize this attenuation and phase distortion, the currents of all frequencies of the audio spectrum can be forced to flow through the same media and have the same uniform dielectric around them. One method that has been used to accomplish this is to limit the gauge of the conductors so that the skin depth of the currents at the highest frequencies completely penetrate the conductors. The optimum gauge using copper has been analytically and empirically determined to be between 0.5 and 1 mm diameter. In order to further reduce inductance and resistance without increasing skin effects larger diameter hollow conductors have been used in some interconnects. These constructions confine the current flow at all frequencies mechanically.
The veiling affects are more noticeable in the high-frequency audio range although the defocusing effect of transients can give the impression of affecting mid-frequencies. Two manufacturers have eliminated the veils and defocusing in their products by inserting low-pass filters in the interconnect. This approach does seem to eliminate the veils, but at the expense of high-frequency response. Reproduction of cymbals and other high-frequency material tends to be compromised in the process.
Other interconnects locate hollow insulating tubing around and between the conductors to lower capacitance and capture more air in the dielectric. The capacitance and dielectric absorption can both be reduced by creating air spaces around the conductors.
There are currently five types of geometries utilized in high-fidelity single-ended and balanced cables, the coaxial, the twisted-pair, the parallel-pair, the woven and the helical-pair. The simplest geometry used in single-ended interconnects is a coaxial arrangement. This geometry locates the forward signal conductor at the concentric center of an overall shield. The shield is usually composed of a large number of fine wires woven into a tubular shape or wrapped in a helical fashion. The return signal current flows in the overall shield. To lower the capacitance of this interconnect, low-dielectric constant materials have been utilized for insulation of the center conductor. Air pockets have also been introduced by spiraling a solid or tubular insulator around the center conductor. The disadvantage of the coaxial arrangement is that there is significant capacitive coupling between the entire surface of the inner conductor and the inside surface of the overall shield conductor. The current distribution in the shield is also very different than that in the inner conductor. Only one successful high-fidelity audio cable currently utilizes this geometry. This particular cable utilizes a center conductor that is tubular in geometry, which reduces skin-effect. It also utilizes air-filled dielectric material between the overall shield and the center conductor. This interconnect has the disadvantage that the large adjacent surfaces of the inner conductor and overall shield creates a highly capacitive cable. Most types of coaxial constructions tend to have poor phase linearity when used with consumer electronics.
A typical twisted-pair is illustrated in FIG. 1a. The twisted-pair geometry has the disadvantage that the close proximity of the conductors 101 tends to decrease inductance at the expense of increasing capacitance. The shield 102 is usually composed of a large number of fine wires woven into a tubular shape or wrapped in a helical fashion. Sometimes the shield is a spirally wrapped foil or combination of braid and foil. Because there is dielectric material directly between the conductors but air around both of them as a result of inserting air-filled materials, this creates a non-uniform lossy dielectric. Since the EM fields have the highest magnitude directly between the two signal conductors, the dielectric absorption and losses of the insulating material can affect the signal quality. To minimize this effect, some interconnects utilize expanded Teflon.TM. which contains a high-percentage of air or other inert gases to insulate the conductors. This achieves a very low-capacitance cable. However, these interconnects have been shown empirically to have lack of focus in the stereo image. Some filtering is required in these interconnects to eliminate the "veils" as well. The veils are reduced as compared to an interconnect utilizing standard un-expanded Teflon.TM.. However, they are still present, it is believed, due to dielectric absorption effects.
Some versions of twisted-pairs are unshielded and therefore achieve significant air dielectric around the conductors to minimize capacitance, however they are more susceptible to noise. Versions with overall shields are not as susceptible to noise but the capacitance tends to be increased due to elimination of air in the dielectric and coupling of the signal conductors to the shield, causing the high-frequencies to roll-off.
Illustrated in FIG. 1b, the helical-pair geometry used in some interconnects comprises a helical wrap of two signal conductors 103 around a large Teflon.TM. tubing core 104. In some cases, small hollow tubing is also helically wrapped around the tubing core to act as spacers, holding the conductors in place. Air dielectric is present on three sides of the conductors in this construction. Because the two conductors are spaced apart as they wrap around the core, this makes the interconnect susceptible to noise. Therefore, these types of interconnects typically have an overall shield 105 to reduce susceptibility. The shield is generally spaced away from the signal conductors by additional insulating material. This type of interconnect has three disadvantages. First, only three sides of the signal conductors are adjacent to the air dielectric. Second, both signal conductors capacitively couple to the overall shield, which in most cases is spaced fairly close, causing increased capacitance. Third, the conductors are generally located on opposite sides of the tubing core, causing the interconnect to be inflexible and susceptible to mechanical damage. Bending the cable causes one signal conductor to stretch as the other is compressed.
FIG. 1c illustrates a woven geometry. The braided weave causes air to be interlaced between the conductors reducing the capacitance. The woven geometry has the disadvantage that it is susceptible to noise and that the forward and return paths are not identical. Noise susceptibility is high because the geometry is not twisted and therefore common-mode noise is not effectively canceled. No combination of two of the three signal conductors forms a twisted-pair geometry. The absence of an overall shield also tends to increase noise susceptibility. All three conductors are used for signal transmission in the woven geometry. This causes the geometry of the forward signal path to be different than the return signal path. Due to the differing geometry of the forward and return paths, electromagnetic field coupling between conductors can cause undesired currents to be induced. The effect has been empirically shown to be a defocusing of the stereo image forming a "musical soup", particularly noticeable at high frequencies.
Parallel-pair constructions suspend two parallel conductors within an overall shield. One version of the parallel-pair interconnect is shown in U.S. Pat. No. 4,954,095 of Cogan. Two parallel hollow solid copper conductors are shown being supported using spacers that suspend the signal conductors in an air dielectric. This creates air-filled spaces 180 degrees around the conductors and between the signal conductors and the overall shield. This construction minimizes capacitance and creates a uniform low-loss dielectric. It also has the advantage of utilizing a single identical conductor each for forward and return currents. The disadvantage of this interconnect is that it has limited flexibility in one dimension and is inflexible in the other dimension. The use of solid conductors causes the interconnect to be quite stiff and must be "formed" into the desired shape. It is also difficult to terminate this cable to conventional RCA and XLR type audio connectors. Manufacture of this cable is also very difficult to mechanize, increasing the cost. Constructions such as that utilized in Cogan use spacers to suspend the conductors in air which approaches the ideal combination of uniform air dielectric, low capacitance between combinations of conductors and shield, and low inductance. Low inductance is achieved in this case by using large gauge hollow single conductor tubing as opposed to twisting the signal conductors together.
The disadvantage of this and other existing spacer constructions is that they tend to be very inflexible, fragile and are difficult to terminate to RCA or XLR connectors.
Another version of the parallel-pair is shown in U.S. Pat. No. 4,767,890 of Magnan. The Magnan cable is actually a combination of two helically wrapped singles to form a parallel pair. This interconnect succeeds in creating air gaps around the signal conductors which creates a uniform low-loss dielectric. Skin-effects are eliminated by utilizing a multiplicity of small individually insulated conductors to convey the signal. The conductors in the Magnan cable are arranged in such a way as to approximate two tubular conductors that are parallel to each other. This technique has the disadvantage that it allows differences in group-delay to occur between conductors and non-linearities in phase response due to non-uniform current-sharing across all conductor strands. It is preferred to utilize a single identical conductor for each signal direction to avoid current-sharing and the need to electrically equalize all conductor lengths. The Magnan cable has the added disadvantage of having restricted mechanical flexibility, particularly in one dimension. It is also difficult to terminate to RCA and XLR connectors.
Yet another example of a parallel-pair that is suspended in a gaseous dielectric is described in U.S. Pat. No. 2,034,033 of E. I. Green and H. E. Curtis. Green and Curtis describe a transmission-cable composed of a series of dielectric washers that hold two conductors at a specified spacing from each other inside an overall circular shield. The two conductors are twisted helically about the axis of the shield. The spacing relationships between the conductors and shield are critical to Green and Curtis for achieving low attenuation at high frequencies. The disadvantage of this construction is that the conductors cannot maintain their spacing relationship when the cable is severely flexed. The cable cannot withstand severe bends without compromising the conductor spacing or otherwise displacing the conductors from their optimum positions. The cable of Green and Curtis is also inflexible because the outer jacketing does not contain features which allow severe bending, such as corrugations.
None of the interconnect constructions described achieve an optimum balance of low capacitance and inductance, uniform low-loss dielectric, compatibility with standard RCA or XLR plugs, and mechanical flexibility that allows severe bending without altering the relationship of the conductors.